Direct conversion receiver architecture

ABSTRACT

A direct downconversion receiver architecture having a DC loop to remove DC offset from the signal components, a digital variable gain amplifier (DVGA) to provide a range of gains, an automatic gain control (AGC) loop to provide gain control for the DVGA and RF/analog circuitry, and a serial bus interface (SBI) unit to provide controls for the RF/analog circuitry via a serial bus. The DVGA may be advantageously designed and located as described herein. The operating mode of the VGA loop may be selected based on the operating mode of the DC loop, since these two loops interact with one another. The duration of time the DC loop is operated in an acquisition mode may be selected to be inversely proportional to the DC loop bandwidth in the acquisition mode. The controls for some or all of the RF/analog circuitry may be provided via the serial bus.

This application claims the benefit of provisional U.S. Application Ser.No. 60/269,623, entitled “AUTOMATIC GAIN CONTROL FOR DIRECT CONVERSIONRF ARCHITECTURE,” filed Feb. 16, 2001, which is incorporated herein byreference in its entirety for all purposes; and is a continuation ofUtility application Ser. No. 10/034,734 entitled “DIRECT CONVERSIONRECEIVER ARCHITECTURE” and filed on Dec. 21, 2001 now U.S. Pat. No.7,076,225.

BACKGROUND

1. Field

The present invention relates generally to electronic circuits, and morespecifically to a direct downconversion receiver architecture for use ina wireless (e.g., CDMA) communication system.

2. Background

In a CDMA system, data to be transmitted is initially processed togenerate a radio frequency (RF) modulated signal that is more suitablefor transmission over a wireless communication channel. The RF modulatedsignal is then transmitted over the communication channel to one or moreintended receivers, which may be terminals in the CDMA system. Thetransmitted signal is affected by various transmission phenomena, suchas fading and multipath. These phenomena result in the RF modulatedsignal being received at the terminals at a wide range of signal powerlevels, which may be 100 dB or more.

At a given terminal, the transmitted signal is received, conditioned,and downconverted to baseband by a receiver front-end unit.Conventionally, the frequency downconversion from RF to baseband isperformed with a heterodyne receiver that includes multiple (e.g., two)frequency downconversion stages. In the first stage, the received signalis downconverted from RF to an intermediate frequency (IF) wherefiltering and amplification are typically performed. And in the secondstage, the IF signal is then downconverted from IF to baseband whereadditional processing is typically performed to recover the transmitteddata.

The heterodyne receiver architecture provides several advantages. First,the IF frequency may be selected such that undesired inter-modulation(IM) products, which result from non-linearity in the RF and analogcircuitry used to condition and downconvert the received signal, may bemore easily filtered. Second, multiple filters and variable gainamplifier (VGA) stages may be provided at RF and IF to provide thenecessary filtering and amplification for the received signal. Forexample, an RF amplifier may be designed to provide 40 dB of gain rangeand an IF amplifier may be designed to provide 60 dB of gain range,which would then collectively cover the 100 dB of dynamic range for thereceived signal.

For certain applications, such as cellular telephone, it is highlydesirable to simplify the receiver design to reduce size and cost.Moreover, for mobile applications such as cellular telephone, it ishighly desirable to reduce power consumption to extend battery lifebetween recharges. For these applications, a direct downconversionreceiver (which is also known as a homodyne receiver or a zero-IFreceiver) may provide these desired benefits because it uses only onestage to directly downconvert the received signal from RF to baseband.

Several challenges are encountered in the design of a directdownconversion receiver. For example, because there is no IF signal inthe direct downconversion receiver, the (e.g., 60 dB) gain rangenormally provided by the IF amplifier in the heterodyne receiver wouldneed to be provided instead at either RF or baseband in the directdownconversion receiver. To avoid placing additional requirements on theRF circuitry and to reduce cost and circuit complexity, this IF gainrange may be provided at baseband. However, if the baseband gain rangeis provided digitally after analog-to-digital conversion, then thebaseband signal provided to the analog-to-digital converter (ADC) wouldhave smaller amplitude since the gain is provided digitally after theADC. DC offset in the baseband signal would then become a more criticalconsideration in the direct downconversion receiver because the basebandsignal amplitude is smaller, and the DC offset may be a much largerpercentage of the signal amplitude.

There is therefore a need in the art for a direct downconversionreceiver architecture capable of providing the required signal gain andDC offset correction.

SUMMARY

Aspects of the invention provide a direct downconversion receiverarchitecture having a DC loop to remove DC offset from the signalcomponents prior to and after the analog-to-digital conversion, adigital variable gain amplifier (DVGA) to provide a range of gains, anautomatic gain control (AGC) loop to provide gain control for theRF/analog circuitry and the DVGA, and a serial bus interface (SBI) unitto provide controls for the RF/analog circuitry using a compact serialinterface.

In an aspect, a DVGA is provided for use in the direct downconversionreceiver. The DVGA can provide the required range of gains needed toaccount for all or a portion of the total dynamic range of the receivedsignal (i.e., the portion not accounted for by the RF/analog circuitry).The design of the DVGA and the placement of the DVGA within the directdownconversion receiver architecture may be advantageously implementedas described herein.

In another aspect, the operating mode of the VGA loop is selected basedin part on the operating mode of the DC loop. Since these two loopsoperate (directly or indirectly) on the same signal components, theyinteract with one another. Techniques are provided herein for a loop tosignal an event that may impact the performance of the other loop, sothat the other loop can appropriately handle the event to minimizeperformance degradation. For example, if the DC loop is operated in anacquisition mode to quickly remove large DC offsets, large DC spikes canbe produced that may have various deleterious effects on the AGC loop,then this event is triggered and the AGC loop may then be operated in alow gain mode or frozen altogether to minimize the effects of the DCspikes on the operation of the AGC loop.

In yet another aspect, the duration of time the DC loop is operated inthe acquisition mode is inversely proportional to the bandwidth of theDC loop in the acquisition mode. The DC loop bandwidth is designed to bewider in the acquisition mode to allow the DC loop to more quicklyrespond to and remove DC offset in the signal components. However, thewider loop bandwidth also results in more loop noise generated by the DCloop. To limit the amount of total noise (which includes the DC spike tobe corrected and the loop noise) and still allow the DC loop to operateat high bandwidth, the time duration in which the DC loop operates inthe acquisition mode may be set inversely proportional to the loopbandwidth. Since a wider loop bandwidth is able to more quickly correctfor the DC offset, a shorter amount of time spent in the acquisitionmode improves performance.

In yet another aspect of the invention, the controls for some or all ofthe RF/analog circuitry are provided via a serial bus. The use of astandard serial bus to control RF/analog functions provides manyadvantages, such as reduced pin count, simplified board layout, reducedcost, and so on. The serial bus may be designed with various features tomore effectively provide the controls. For example, multiple hardwarerequest channels may be supported (e.g., one channel for each circuit tobe individually controlled), each channel may be associated with arespective priority, and messages may be transmitted on each channelusing a number of possible data transfer modes.

Various aspects and embodiments of the invention are described infurther detail below. The invention further provides methods, digitalsignal processors, receiver units, and other apparatuses and elementsthat implement various aspects, embodiments, and features of theinvention, as described in further detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, nature, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 is a block diagram of an embodiment of a receiver unit capable ofimplementing various aspects and embodiments of the invention;

FIG. 2A is a block diagram of an embodiment of a direct downconverter;

FIG. 2B is a block diagram of an embodiment of a DC offset canceller;

FIG. 3 is a block diagram of an embodiment of a digital variable gainamplifier (DVGA);

FIG. 4A is a block diagram of an AGC loop unit;

FIG. 4B is a block diagram of an AGC control unit; and

FIG. 4C is a diagram of an example gain transfer function for theRF/analog circuits.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of an embodiment of a receiver unit 100capable of implementing various aspects and embodiments of theinvention. Receiver unit 100 may be implemented within a terminal or abase station of a wireless (e.g., CDMA) communication system. Forclarity, various aspects and embodiments of the invention are describedfor a receiver implementation in a terminal. Also for clarity, specificdesign values are provided herein, but other design values may also beused and are within the scope of the invention.

In FIG. 1, one or more RF modulated signals transmitted from one or moretransmitters (e.g., base stations, GPS satellites, broadcast stations,and so on) are received by an antenna 112 and provided to an amplifier(Amp) 114. Amplifier 114 amplifies the received signal with a particulargain to provide an amplified RF signal. Amplifier 114 may comprise oneor more low noise amplifier (LNA) stages designed to provide aparticular range of gains and/or attenuation (e.g., 40 dB from maximumgain to attenuation). The specific gain of amplifier 114 may bedetermined by a gain control message provided by a serial bus interface(SBI) unit 150 via a serial bus 152. The amplified RF signal is thenfiltered by a receive filter 116 to remove noise and spurious signals,and the filtered RF signal is provided to a direct downconverter 120.

Direct downconverter 120 performs direct quadrature downconversion ofthe filtered RF signal from RF to baseband. This may be achieved bymultiplying (or mixing) the filtered RF signal with a complex localoscillator (LO) signal to provide a complex baseband signal. Inparticular, the filtered RF signal may be mixed with an inphase LOsignal to provide an inphase (I) baseband component and mixed with aquadrature LO signal to provide a quadrature (Q) baseband component. Themixer used to perform the direct downconversion may be implemented withmultiple stages that may be controlled to provide different gains, asdescribed below. In this case, the specific gain to be provided by themixer may also be determined by another gain control message provided bySBI unit 150 via serial bus 152, as shown in FIG. 1. The I and Qbaseband components are then provided to one or more analog-to-digitalconverters (ADCs) 122.

ADCs 122 digitize the I and Q baseband components to provide I and Qsamples, respectively. ADCs 122 may be implemented with various ADCdesigns, such as with sigma-delta modulators capable of filtering andthen over-sampling the I and Q baseband components at multiple (e.g.,16) times the chip rate of the baseband components (which is 1.2288 Mcpsfor IS-95). The over-sampling allows the ADCs to provide a higherdynamic range and further allows the I and Q samples to be provided withfewer number of bits for a given precision. In a specific embodiment,ADCs 122 provide 2-bit I and Q samples at 16 times the chip rate (i.e.,chip×16). Other types of ADCs may also be used and are within the scopeof the invention. The I and Q samples are provided from ADCs 122 to adigital filter 124.

Digital filter 124 filters the I and Q samples to provide filtered I andQ samples, respectively. Digital filter 124 may perform any number offunctions such as image rejection filtering, baseband pulse-matchedfiltering, decimation, sample rate conversion, and so on. In a specificembodiment, digital filter 124 provides 18-bit filtered I and Q samplesat chip×8 to a DC offset canceller 130.

DC offset canceller 130 removes DC offset in the filtered I and Qsamples to provide DC offset corrected I and Q samples, respectively. Ina specific embodiment, DC offset canceller 130 implements two DC offsetcorrection loops that attempt to remove DC offsets at two differentlocations in the received signal path—one at baseband after thefrequency downconversion by direct downconverter 120 and another afterthe digital filtering by filter 124. The DC offset correction isdescribed in further detail below.

A digital variable gain amplifier (DVGA) 140 then digitally amplifiesthe DC offset corrected I and Q samples to provide I and Q data forsubsequent processing by a digital demodulator 144. In a specificembodiment, DVGA 140 provides 4-bit I and Q data at chip×8.

Digital demodulator 144 demodulates the I and Q data to providedemodulated data, which may then be provided to a subsequent decoder(not shown in FIG. 1). Demodulator 144 may be implemented as a rakereceiver that can concurrently process multiple signal instances in thereceived signal. For CDMA, each finger of the rake receiver may bedesigned to (1) rotate the I and Q data with a complex sinusoidal signalto remove frequency offset in the I and Q data, (2) despread the rotatedI and Q data with a complex pseudo-random noise (PN) sequence used atthe transmitter, (3) decover the despread I and Q data with thechannelization code (e.g., a Walsh code) used at the transmitter, and(4) data demodulate the decovered I and Q data with a pilot recoveredfrom the received signal. Digital filter 124, DC offset canceller 130,DVGA 140, and digital demodulator 144 may be implemented within one ormore integrated circuits (ICs), e.g., within a single digital signalprocessor.

An automatic gain control (AGC) loop unit 142 receives the I and Q datafrom DVGA 140 and a DC_loop_mode signal from DC offset canceller 130,and provides the gains for various variable gain elements withinreceiver unit 100. In an embodiment, the gains for amplifier 114 anddirect downconverter 120 are provided to SBI unit 150, which thenprovides the appropriate gain control messages to these elements viaserial bus 152. The gain for DVGA 140 is provided directly to the DVGAafter taking into account the delay from the RF signal input to theinput of the DVGA. AGC loop unit 142 provides the appropriate gains foramplifier 114, direct downconverter 120, and DVGA 140 such that thedesired amplitude for the I and Q data is achieved. The AGC loop isdescribed in further detail below.

A controller 160 directs various operations of receiver unit 100. Forexample, controller 160 may direct the operation of the DC offsetcancellation, the AGC loop, the DVGA, the SBI, and so on. A memory 162provides storage for data and program codes for controller 160.

In a typical receiver design, the conditioning of the received signalmay be performed by one or more stages of amplifier, filter, mixer, andso on. For example, the received signal may be amplified by one or moreLNA stages. Also, filtering may be provided before and/or after the LNAstages, and is also typically performed after the frequencydownconversion. For simplicity, these various signal conditioning stagesare lumped together into the blocks shown in FIG. 1. Other RF receiverdesigns may also be used and are within the scope of the invention.Amplifier 114, direct downconverter 120, and ADCs 122 form an RFfront-end unit for the direct downconversion receiver.

The resolution of the I and Q samples at various signal processingblocks in FIG. 1 are provided for illustration. Different number of bitsof resolution and different sample rates may also be used for the I andQ samples, and this is within the scope of the invention.

DC Offset Correction

FIG. 2A is a block diagram of a direct downconverter 120 a, which is aspecific embodiment of direct downconverter 120 in FIG. 1. Within directdownconverter 120 a, the filtered RF signal from receive filter 116 isprovided to a mixer 212, which also receives a (complex) LO signal froma local oscillator 218. The frequency of the LO signal may be controlledby a frequency control signal (which may be provided via serial bus 152or some other signal lines) and is set to the center frequency of the RFmodulated signal being recovered. Mixer 212 then performs quadraturedownconversion of the filtered RF signal with the complex LO signal toprovide inphase and quadrature components, which are then provided to asummer 214.

A converter 220 receives a digital DC offset control, which may beprovided by DC offset canceller 130 via serial bus 152 and is denoted asSBI DC control in FIG. 2A. Converter 220 then performs digital-to-analogconversion of the digital control to generate DC offset control valuesof DC1I and DC1Q for the inphase and quadrature components,respectively. In an embodiment, these values are used to control thebias current of mixer 212 such that the DC offset in the signalcomponents may be adjusted indirectly.

Analog circuitry 222 receives an analog DC offset control, which may beprovided by DC offset canceller 130 via a dedicated signal line anddenoted as coarse DC offset in FIG. 2A. Analog circuitry 222 thenperforms filtering and possibly level shifting and scaling to generateDC offset values of DC2I and DC2Q for the inphase and quadraturecomponents, respectively. Summer 214 then subtracts the DC offset valuesof DC2I and DC2Q from the inphase and quadrature components,respectively. The output components from summer 214 are then filteredand amplified by a lowpass filter/amplifier 216 to provide the I and Qbaseband components.

FIG. 2B is a block diagram of a DC offset canceller 130 a, which is aspecific embodiment of DC offset canceller 130 in FIG. 1. DC offsetcanceller 130 a includes summers 232 a and 232 b, DC loop control units234 a and 234 b, an SBI DC offset controller 240, and a DC loopcontroller 242. In an embodiment, the DC offset correction is performedseparately for the I and Q samples. Thus, summers 232 a and 232 b and DCloop control units 234 a and 234 b each includes two elements, one toprocess the I samples and another to process the Q samples.

The filtered I and Q samples from digital filter 124 are provided tosummer 232 a, which removes fixed DC offset values of DC3I and DC3Q fromthe I and Q samples, respectively. Summer 232 a may be used to remove DCoffset that is static (e.g., caused by circuit mismatch and so on). TheI and Q outputs from summer 232 a are then provided to summer 232 b,which further removes DC offset values of DC4I and DC4Q (which areprovided by DC loop control unit 234 b) from these I and Q outputs toprovide the DC offset corrected I and Q samples.

DC loop control unit 234 a receives the I and Q outputs from summer 232a, determines the DC offsets in these outputs, and provides the coarseDC control to analog circuitry 222 within direct downconverter 120 a. DCloop control unit 234 b similarly receives the I and Q outputs fromsummer 232 b, determines the DC offsets in these outputs, and providesthe DC offset values of DC4I and DC4Q to summer 234 b. Each DC loopcontrol unit 234 is implemented with a gain element 236 coupled to anaccumulator 238. Gain element 236 multiplies the input I or Q samplewith a particular gain (DC gain 1 for unit 234 a and DC gain 2 for unit234 b) selected for that loop. Accumulator 238 then accumulates thescaled I or Q sample to provide the DC offset control for that loop.

Summer 214 within direct downconverter 120 a and DC loop control unit234 a implement a coarse-grain DC loop that removes DC offset in thebaseband components after the direct downconversion by mixer 212. Summer232 b and DC loop control unit 234 b implement a fine-grain DC loop thatremoves DC offset that still remains after the coarse-grain DC loop. Astheir names imply, the fine-grain DC loop has higher resolution than thecoarse-grain DC loop.

SBI DC offset controller 240 periodically determines the SBI DC offsetcontrol based on various factors such as temperature, the gains ofamplifier 114 and mixer 212, time, drift, and so on. The SBI DC offsetcontrol is then provided via serial bus 152 to converter 220, which thengenerates the corresponding DC offset control values of DC1I and DC1Qfor mixer 212.

An implementation of the DC offset correction for a directdownconversion receiver, such as the one shown in FIG. 1, is describedin further detail in U.S. patent application Ser. No. 10/139,205,entitled “Direct Current Offset Cancellation for Mobile Station ModemsUsing Direct Downconversion,” filed May 2, 2002, which is incorporatedherein by reference.

The four sets of DC offset values (DC1I and DC1Q, DC2I and DC2Q, DC3Iand DC3Q, and DC4I and DC4Q) represent four different mechanisms thatmay be used individually or in combination to provide the required DCoffset correction for the direct downconversion receiver. Thecoarse-grain DC loop (which provides the values of DC2I and DC2Q) andthe fine-grain DC loop (which provides the values of DC4I and DC4Q) maybe operated to dynamically remove DC offset in the I and Q signalcomponents. Summer 232 a (which subtracts the values of DC3I and DC3Q)may be operated to remove static DC offset. And SBI DC offset controller240 (which provides the values of DC1I and DC1Q) may be used to removedynamic and/or static DC offset in the signal components.

In an embodiment, the coarse-grain and fine-grain DC loops each supportstwo operating modes—an acquisition mode and a tracking mode. Theacquisition mode is used to more quickly remove large DC offset that mayhave been introduced in the signal components as a result of (1) a stepchange in the gains of the RF/analog circuitry such as amplifier 114and/or mixer 212, or (2) the overall DC loop performing a periodic DCupdate, which may result in new values of DC1 and/or DC3 being providedto mixer 212 and/or summer 232 a, or (3) or any other reasons,respectively. The tracking mode is used to perform the DC offsetcorrection in a normal manner, and its response is slower than that ofthe acquisition mode. Different or additional operating modes may alsobe supported, and this is within the scope of the invention. Theacquisition and tracking modes may correspond to two different DC loopgain values for DC gain 1 and to two different DC loop gain values forDC gain 2.

For simplicity, the coarse-grain and fine-grain DC loops arecollectively referred to as simply the “DC loop”. The DC_loop_modecontrol signal indicates the DC loop's current operating mode. Forexample, the DC_loop_mode control signal may be set to logic high toindicate that the DC loop is operating in the acquisition mode and tologic low to indicate that it is operating in the tracking mode.

Digital VGA

An aspect of the invention provides a DVGA for use in a directdownconversion receiver. The DVGA can provide the required range ofgains needed to account for all or a portion of the total dynamic rangefor the received signal (i.e., the portion not accounted for by theRF/analog circuitry). The DVGA's gain range may thus be used to providethe gain previously provided at intermediate frequency (IF) in aheterodyne receiver. The design of the DVGA and the placement of theDVGA within the direct downconversion receiver architecture may beadvantageously implemented as described below.

FIG. 3 is a block diagram of a DVGA 140 a capable of providing digitalbaseband gain for I and Q samples. DVGA 140 a is a specific embodimentof DVGA 140 in FIG. 1.

Within DVGA 140 a, the DC offset corrected I and Q samples from thepreceding DC offset canceller 130 are provided to a multiplexer (MUX)312 and a truncation unit 320. To minimize hardware, only one digitalmultiplier 316 is used to perform the gain multiplication for both I andQ samples in a time division multiplexed (TDM) manner. Thus, multiplexer312 alternately provides an I sample and then a Q sample (as determinedby an IQ_sel control signal) to multiplier 316 via an AND gate 314. TheIQ_sel control signal is simply a square wave at the I and Q sample rate(e.g., chip×8) and having the appropriate phase (e.g., logic low for theI samples). AND gate 314 performs an AND operation of the I or Q samplewith a DVGA_enb control signal, which is set to logic high if the DVGAis enabled and set to logic low if the DVGA is bypassed. The DVGA may bebypassed, for example, if the DVGA's gain range is not needed or if thegain range is provided with analog circuitry (e.g., a variable gainamplifier). AND gate 314 thus passes the sample to multiplier 316 if theDVGA is enabled and provides a zero otherwise. The zero reduces powerconsumption by the subsequent circuitry by eliminating transitions thatconsume power in CMOS circuits.

Multiplier 316 multiplies the I or Q sample from AND gate 314 with again from a register 344 and provides the scaled (or amplified) sampleto a truncation unit 318. In a specific embodiment, multiplier 316 isoperated at twice the sample rate, which is chip×16 for I/Q sample rateof chip×8. In a specific embodiment, for CDMA and GPS, the input I and Qsamples have 18 bits of resolution with 10 bits of resolution to theright of the binary point (i.e., 18Q10), the gain has 19 bits ofresolution with 12 bits of resolution to the right of the binary point(i.e., 19Q12), and the scaled samples have 37 bits of resolution with 22bits of resolution to the right of the binary point (i.e., 37Q22). In aspecific embodiment, for digital FM or DFM, the input I and Q sampleshave a resolution of 18Q6, the gain has a resolution of 19Q12, and thescaled samples have a resolution of 37Q18. Truncation unit 318 truncatesthe (e.g., 18) least significant bits (LSBs) of each scaled sample andprovides the truncated sample (which has a resolution of 18Q4 forCDMA/GPS and 18Q0 for DFM) to one input of a multiplexer 322.

For certain operating modes of the receiver, the digital scaling by DVGA140 a is not needed and the input I and Q samples may be passed to theDVGA's output without any scaling (after the appropriate processing toobtain the desired output data format). Truncation unit 320 truncatesthe (e.g., 6) LSBs of each input sample and provides the truncatedsample to the other input of multiplexer 322. Truncation unit 320ensures that the output I and Q data have the same resolution regardlessof whether the DVGA is enabled or bypassed.

Multiplexer 322 then provides the truncated sample from eithertruncation unit 318 or 320 depending on whether the DVGA is enabled orbypassed, respectively, which is determined by the DVGA_enb controlsignal. The selected sample is then provided to a saturation unit 324,which saturates the sample to the desired output data format, e.g., aresolution of 8Q4 for CDMA/GPS and 8Q0 for DFM. The saturated sample isthen provided to a delay element 326 and to one input of a register 328.Delay element 326 provides one-half sample period of delay to align theI and Q data (which have been skewed by one-half sample period toimplement the time division multiplexing for multiplier 316) andprovides the delayed I sample to the other input of register 328.Register 328 then provides the I and Q data, with the timing aligned tothe IQ_sel control signal. For CDMA/GPS, the four most significant bits(MSBs) of the I and Q data (i.e., with a resolution of 4Q0) is sent tothe next processing block. And for DFM, the I and Q data (i.e., with aresolution of 8Q0) is sent directly to an FM processing block.

Receiver unit 100 may be used for various applications such as toreceive data from a CDMA system, a GPS system, a digital FM (DFM)system, and so on. Each such application may be associated with arespective received signal having some particular characteristics andrequiring some particular gain. As shown in FIG. 3, the three differentgains to be used for CDMA, GPS, and DFM are provided to a multiplexer332. One of the gains is then selected based on a Mode_sel controlsignal. The selected gain is then provided to a gain scaling and offsetunit 334, which also receives a gain offset.

Gain scaling and offset unit 334 scales the selected (CDMA, GPS, or DFM)gain with an appropriate scaling factor such that the desired gainresolution is achieved. For example, the CDMA gain may be provided witha fixed number of bits (e.g., 10 bits) that cover one of severalpossible gain ranges (e.g., 102.4 dB and 85.3 dB gain ranges for the10-bit CDMA gain), depending on the particular mode used for CDMA. Thescaling factor is then selected such that the scaled CDMA gain has thesame gain resolution (e.g., 0.13 dB) regardless of the particular modeused for CDMA. Gain scaling and offset unit 334 further subtracts thegain offset from the scaled gain. The gain offset is determined based ona setpoint selected for ADCs 122, which in turn determines the averagepower of the I and Q baseband components provided to the ADCs. The gainoffset may be a programmable value having the same resolution as thescaled gain, and may be provided by controller 160.

A multiplexer 336 receives the scaled and offsetted gain from unit 334and an override gain and provides one of the gains (based on aGain_override control signal) to a saturation unit 338. The overridegain may be used instead of the gain from the VGA loop, if it is desiredto bypass the VGA loop. Saturation unit 338 then saturates the receivedgain (e.g., to 9 bits) to limit the range of the saturated gain (e.g.,to 68.13 dB of total gain range for 9 bits, with 0.133 dB of resolutionfor each bit). An AND gate 340 then performs an AND operation on thesaturated gain with the DVGA_enb control signal, and passes thesaturated gain to a dB-to-linear look-up table (LUT) 342 if the DVGA isenabled or a zero otherwise (again, to reduce power consumption by thesubsequent circuitry).

In an embodiment, the AGC loop provides the gain value (e.g., the CDMAgain) in logarithm (dB) format. The dB gain value may be used to mimicthe characteristics of RF/analog variable gain circuits, which typicallyhave log (or log-like) transfer functions for gain versus control value.Secondly, the receive gain is used as an estimate for the requiredtransmit power in a CDMA phone call and to report the receive power tothe base station when requested. These estimations are traditionallydone in dB given the wide dynamic range of the received signal. However,since a linear digital multiplier 316 is used to provide the basebandgain multiplication, the dB gain value is translated to a linear gainvalue. Look-up table 342 performs the dB-to-linear translation based ona formula, which may be expressed as:Y(linear)=10^(X/20),  Eq (1)where Y is the linear gain value from the look-up table and X is anattenuation value, which may be defined as:X=−(Z(dB)+offset),  Eq (2)where Z is the dB gain value provided to the look-up table and theoffset in equation (2) may be used to compensate for the truncationperformed in unit 334 (e.g., offset=0.067 dB for a 4-bit truncation).Other techniques for converting dB gain value to linear gain value mayalso be used, and this is within the scope of the invention. The lineargain value from LUT 342 is then clocked by register 344 to align thetiming of the gain value with that of the I or Q sample provided tomultiplier 316.

The AGC loop may also be designed to operate based on linear (instead ofdB) gain values, and this is within the scope of the invention.

Referring back to FIG. 1, DVGA 140 is placed after DC offset canceller130 and outside of the DC loop in direct downconversion receiver 100.This DVGA placement provides several advantages and further avoidsseveral disadvantages. First, if the DVGA is placed within the DC loop,then any DC offset will be amplified by the gain of the DVGA, whichwould then exacerbate the degradation caused by the DC offset. Second,the loop gain of the DC loop would also include the gain of the DVGA,which varies depending on the received signal strength. Since this DCloop gain directly affects (or determines) the bandwidth of the DC loop,the DC loop bandwidth would then vary along with the gain of the DVGA,which is an undesirable effect. The DC loop bandwidth may be maintainedapproximately constant by dynamically changing the DC loop gain (i.e.,DC gains 1 and 2 within DC loop units 234 a and 234 b) in an inversemanner to any change in the DVGA gain, so that the overall DC loop gainis maintained constant. However, this would further complicate thedesign of the DC offset correction mechanisms. Moreover, the residual DCoffset is variable when referenced to the actual signal power.

By advantageously placing DVGA 140 after DC offset canceller 130 andoutside of the DC loop, the DC offset correction by the DC loop may bedecoupled from the signal gain scaling by the DVGA. Moreover,implementation of the DVGA in the digital domain after ADCs 122 furthersimplify the design of the RF/analog circuitry, which may lead toreduced cost for the direct downconversion receiver. Since the digitalgain is provided after ADCs 122, the amplitude of the signal componentsprovided to the ADCs could potentially be smaller, which would thenrequire greater dynamic range for the analog-to-digital conversionprocess so that the ADC noise does not significantly degrade the SNR ofthe quantized I and Q samples. ADCs with high dynamic range may beprovided by over-sampling sigma-delta modulators, as is known in theart.

Automatic Gain Control

FIG. 4A is a block diagram of an AGC loop unit 142 a, which is aspecific embodiment of AGC loop unit 142 in FIG. 1. Within AGC loop unit142 a, the I and Q data is provided to a received signal strengthindicator (RSSI) 412, which estimates the signal strength of thereceived signal. The received signal strength, RSS, may be estimated asfollows:

$\begin{matrix}{{{RSS} = {\sum\limits_{i}^{N_{E}}\;\left\{ {{I^{2}(i)} + {Q^{2}(i)}} \right\}}},} & {{Eq}\mspace{14mu}(3)}\end{matrix}$where I(i) and Q(i) represent the I and Q data for the i-th sampleperiod, and N_(E) is the number of samples to be accumulated to derivethe received signal strength estimate. Other techniques may also be usedto estimate the received signal strength (e.g.,RSS=Σ|I_(F)(i)|+|Q_(F)(i)|). The received signal strength estimate isthen provided to an AGC control unit 414.

FIG. 4B is a block diagram of an AGC control unit 414 a, which is aspecific embodiment of AGC control unit 414 in FIG. 4A. AGC control unit414 a receives the received signal strength estimate, RSS, from RSSI412, the DC_loop_mode control signal from DC offset canceller 130, aNonbypass/hold control signal from gain step control unit 418, a delayedgain step decision from a programmable delay unit 420, and a Freeze_enbcontrol signal (e.g., from controller 160), all of which are describedin further detail below. Based on the received control signals and RSS,AGC control unit 414 a provides an output gain value that is indicativeof the total gain (G_(total)) to be applied to the received signal.

In an embodiment, the AGC loop supports three loop modes—a normal mode,a low gain mode, and a freeze mode. The normal mode is used to provide anominal AGC loop bandwidth, the low gain mode is used to provide asmaller AGC loop bandwidth, and the freeze mode is used to freeze theAGC loop. The low gain and normal modes are associated with AGC loopgain values of AGC gain 1 and AGC gain 2, respectively. The freeze modeis achieved by zeroing out the value provided for accumulation by theAGC loop accumulator. In an embodiment, an additional AGC loop gainvalue of AGC gain 3 is used for jammer detection. AGC gain 3 istypically smaller than AGC gain 2 for the normal mode but larger thanAGC gain 1 for the low gain mode, and is used to detect for the presenceof jammer in the signal components, as described below. Different oradditional modes may also be supported by the AGC loop, and this iswithin the scope of the invention.

As noted above, the DC loop affects the performance of the AGC loop.Thus, in an aspect, the specific AGC loop mode to use is dependent on(i.e., selected based on) the specific DC loop mode currently in use. Inparticular, the normal mode is used for the AGC loop when the DC loop isoperated in the tracking mode, and the low gain or freeze mode is usedfor the AGC loop when the DC loop is operated in the acquisition mode.

As shown in FIG. 4B, the AGC gain 2 for the normal mode and the AGC gain3 for jammer detection are provided to a multiplexer 446, which alsoreceives the Nonbypass/hold control signal. The Nonbypass/hold controlsignal may be used to provide time hysteresis between gain steps (i.e.,the AGC loop is maintained at a given gain step for a particular amountof time (Time 1 or Time 2) before it is allowed to switch to another(higher or lower) gain step.

Multiplexer 446 then provides the AGC gain 2 when the normal mode isselected, which is indicated by the Nonbypass/hold control being set tologic low. Alternatively, multiplexer 446 provides the AGC gain 3 whenjammer detection is to be performed, which is indicated by theNonbypass/hold control being set to logic high. A multiplexer 448receives the AGC gain 1 for the low gain mode and the output frommultiplexer 448 at its two inputs and further receives the DC_loop_modecontrol signal. Multiplexer 448 then provides the AGC gain 1 to amultiplier 442 when the low gain mode is selected for the AGC loop whenthe DC loop is in the acquisition mode, which is indicated by theDC_loop_mode control being set to logic high. Alternatively, multiplexer448 provides the AGC gain 2 or AGC gain 3 to multiplier 442 during thetracking mode, which is indicated by the DC_loop_mode control being setto logic low.

An AND gate 440 receives the received signal strength estimate, RSS, andthe Freeze_enb control. AND gate 440 then provides the RSS to multiplier442 when (1) the DC loop is operated in the tracking mode or (2) the lowgain mode is used for the AGC loop when the DC is operated in theacquisition mode. Alternatively, AND gate 440 provides a zero tomultiplier 442 when the DC loop is operated in the acquisition mode andthe AGC loop is frozen. The zero from AND gate 440 results in anaccumulation of zero by AGC loop accumulator 444 when the AGC loop isfrozen.

Multiplier 442 multiplies the received signal strength estimate, RSS,with the selected AGC gain from multiplexer 448 and provides the resultto AGC loop accumulator 444. Accumulator 444 then accumulates the resultwith the stored value and provides an output gain value that isindicative of the total gain, G_(total), to be applied to the receivedsignal to achieve the desired signal level, which is determined by thegain offset provided to gain scaling and offset unit 334 in FIG. 3. Thistotal gain may be decomposed into two parts—(1) a coarse gain,G_(coarse), for the RF/analog circuitry (e.g., amplifier 114 and mixer212) and (2) a fine gain, G_(fine), for DVGA 140. The total gain for thereceived signal may thus be expressed as:G _(total) =G _(coarse) +G _(fine),  Eq (4)where G_(total), G_(coarse), and G_(fine) are all given in dB.

As shown in FIG. 4B, accumulator 444 also receives the delayed gain stepdecision, which is indicative of the specific discrete gain to be usedfor the RF/analog circuits, as described below. Each discrete gain forthe RF/analog circuits may be associated with a respective set ofmaximum and minimum values for the accumulation, which ensures stabilityin the AGC loop. For the specific discrete gain to be used, as indicatedby the delayed gain step decision, the proper set of maximum and minimumvalues is used for the accumulation by accumulator 444.

Referring back to FIG. 4A, the coarse gain control for the RF/analogcircuits is achieved by (1) mapping the total gain, G_(total), into again step decision by a gain step control unit 418, (2) encoding thegain step decision into the appropriate gain step controls by a rangeencoder 424, (3) formatting the gain step controls into the propermessages by SBI unit 150, (4) sending the messages to the RF/analogcircuits (e.g., amplifier 114 and/or mixer 212) via serial bus 152, and(5) adjusting the gains of the RF/analog circuits based on the messages.The fine gain control is achieved by (1) determining the fine gain,G_(fine), for the DVGA by subtracting the coarse gain, G_(coarse), fromthe total gain, G_(total), and (2) adjusting the gain of the DVGA basedon the fine gain. The derivation of the coarse and fine gains based onthe total gain is described below.

Receiver unit 100 may be designed with amplifier 114 having multiple(e.g., four) stages and mixer 212 having multiple (e.g., two) stages.Each stage may be associated with a specific discrete gain. Depending onwhich stages are turned ON/OFF, different discrete gains may beachieved. The coarse gain then controls the gains of the RF/analogcircuits in coarse discrete steps. The specific discrete gain to be usedfor the RF/analog circuits is dependent on the received signal level,the specific designs of these circuits, and so on.

FIG. 4C is a diagram of an example gain transfer function for theRF/analog circuits (e.g., amplifier 114 and mixer 212). The horizontalaxis represents the total gain, which is inversely related to thereceived signal strength (i.e., higher gain corresponds to smallerreceived signal strength). The vertical axis represents the gain stepdecision provided by gain step control unit 418 based on the total gain.In this specific example design, the gain step decision takes on one offive possible values, which are defined in Table 1.

TABLE 1 Gain Step RF/Analog Decision Circuit State Definition 000 firstlowest gain - all LNA stages are OFF; mixer is in low gain 001 secondsecond lowest gain - all LNA stages are OFF; mixer is in high gain 010third third highest gain - one LNA stage is ON; mixer is in high gain011 fourth second highest gain - two LNA stages are ON; mixer is in highgain 100 fifth highest gain - all three LNA stages are ON, mixer is inhigh gain

As shown in FIG. 4C, hysteresis is provided in the transition betweenadjacent states. For example, while in the second state (“001”), thefirst LNA is not turned ON (to transition to the third state “010”)until the total gain exceeds the L2 Rise threshold, and this LNA is notturned OFF (to transition from the second back to the first state) untilthe total gain falls below the L2 Fall threshold. The hysteresis (L2Rise-L2 Fall) prevents the LNA from being continually turned OFF and ONif the total gain is between or near the L2 Rise and L2 Fall thresholds.

Gain step control unit 418 determines the gain step decision based onthe total gain, a transfer function such as the one shown in FIG. 4C(which is defined by the threshold values), and timing, jammer, andpossibly other information. The gain step decision is indicative of thespecific stages to be turned ON/OFF for amplifier 114 and mixer 212, asdetermined by the transfer function. Referring back to FIG. 4A, gainstep control unit 418 then provides the gain step decision toprogrammable delay element 420 and range encoder 424.

In an embodiment and as shown in FIG. 1, the control to turn ON or OFFeach stage of amplifier 114 and mixer 212 is provided to these circuitsvia serial bus 152. Range encoder 424 receives the gain step decisionand provides the corresponding gain step control for each specificcircuit to be controlled (e.g., one gain step control for amplifier 114and another gain step control for mixer 212). The mapping between thegain step decision and gain step controls may be based on a look-uptable and/or logic. Each gain step control comprises one or more bitsand turns ON/OFF the designated stages within the circuit to becontrolled by that gain step control. For example, amplifier 114 may bedesigned with four stages, and its (2-bit) gain step control may beassociated with four possible values (“00”, “01”, “10”, and “11”) forthe four possible discrete gains for the amplifier. Mixer 212 may bedesigned with two stages, and its (1-bit) gain step control may beassociated with two possible values (“0” and “1”) for the two possiblediscrete gains for the mixer. The gain step controls for amplifier 114and mixer 212 are formatted into the proper messages by SBI unit 150,and these messages are then sent to the circuits via serial bus 152.Range encoder 424 also provides to DC offset canceller 130 a gain stepchange signal that indicates whether or not the gain of the RF/analogcircuitry has changed to a new value or step.

As noted above, the total gain, G_(total), for the received signal maybe decomposed into the coarse gain, G_(coarse), and the fine gain,G_(fine). Moreover, as shown in FIG. 4A, the fine gain is generated bysubtracting the coarse gain from the total gain by a summer 416. Sincethe coarse gain (in the form of the gain step controls) is provided toamplifier 114 and mixer 212 via SBI unit 150 and the serial bus, a delayis introduced between the time the coarse gain is determined by gainstep control unit 418 and the time the coarse gain is actually appliedby the RF/analog circuits. Moreover, processing delay is encountered bythe received signal from the RF circuitry to the DVGA (e.g., especiallydigital filter 124). Thus, to ensure that the coarse gain is applied bythe RF circuits and removed from the DVGA at the same time (i.e., sothat the coarse gain is applied only once to any given data sample), aprogrammable delay is used to delay the coarse gain (as indicated by thegain step decision) before it is applied to DVGA 140.

Programmable delay element 420 provides a particular amount of delay forthe gain step decision. This delay compensates for the delay introducedby SBI unit 150 and the delay of the received signal processing pathfrom the RF circuitry to the DVGA. This delay may be programmed bywriting a delay value to a register. Delay element 420 then provides thedelayed gain step decision.

A coarse gain conversion unit 422 receives the delayed gain stepdecision, which is indicative of a specific discrete gain for theRF/analog circuits, and provides the corresponding coarse gain,G_(coarse), having the proper range and resolution (e.g., the same rangeand resolution as for the total gain from AGC control unit 414). Thecoarse gain is thus equivalent to the gain step decision but is providedin a different format (i.e., the coarse gain is a high-resolution valuewhereas the gain step decision is a digital (ON/OFF) control). The gainstep decision to coarse gain translation may be achieved with a look-uptable and/or logic. The coarse gain is then subtracted from the totalgain by summer 416 to provide the fine gain for the DVGA.

Whenever the gain of the RF/analog circuitry is changed by a coarseamount by switching stages ON and OFF, the phase of the signalcomponents typically rotates by some particular step amount. The amountof phase rotation is dependent on which stages having been switched ONand OFF (as determined by the gain step decision) but is typically afixed value for that particular setting or configuration. This phaserotation may result in degradation in the data demodulation process,until a frequency control loop is able to correct for the phaserotation.

In an embodiment, the gain step decision is mapped to a correspondingrotator phase, which is indicative of the amount of phase rotation inthe received signal components due to the gain indicated by the gainstep decision. The rotator phase is then provided to a rotator withindigital demodulator 144 and used to adjust the phase of the I and Q datato account for the phase rotation introduced by the enabled gain stagesin the RF/analog circuits. The mapping between gain step decision androtator phase may be achieved with a look-up table and/or logic.Moreover, fine resolution may be achieved for the rotator phase (e.g.,5.6° resolution may be achieved with 6 bits for the rotator phase)

DC and AGC Loops Operation

As shown in FIG. 1, the DC loop operates on the filtered I and Q samplesfrom digital filter 124 to remove DC offset, and the AGC loop (via DVGA140) then operates on the DC offset corrected I and Q samples to providethe I and Q data that is then provided to digital demodulator 144. TheAGC loop also controls the gain of the RF/analog circuitry, which inturn affects the amplitude of the I and Q samples operated on by the DCloop. The DC loop may thus be viewed as being embedded within the AGCloop. The operation of the DC loop affects the operation of the AGCloop.

In a direct downconversion receiver, DC offset (both static andtime-varying) has more impact on the signal components because of thesmaller signal amplitude. Large DC offsets (or DC spikes) may beintroduced in the signal components by various means. First, when thegain of the RF/analog circuitry (e.g., amplifier 114 and mixer 212) ischanged in discrete steps by switching ON/OFF stages, large DC offsetsmay be introduced in the signal components due to mismatch in thedifferent stages being switched ON/OFF. Second, large DC offsets mayalso be introduced when the DC loop performs DC offset updates wherebydifferent DC offset values of DC3I and DC3Q are provided to summer 232 aand/or different DC offset values of DC1Q and DC1Q are provided to mixer212 via the serial bus.

Large DC offsets may be removed using various mechanisms of the DC loop(e.g., the coarse-grain and fine-grain loop DC loops). Moreover, largeDC offsets may be more quickly removed by operating the DC loop in theacquisition mode. However, until they are removed, the large DC offsetshave deleterious effects on the signal components and may degradeperformance.

First, any unremoved DC offset in the signal components appears as noise(whose power is equal to the DC offset) after the despreading operationby digital demodulator 144. This noise can degrade performance.

Second, a large DC offset disrupts the performance of the AGC loop inseveral ways. The DC offset adds to the signal components and results incombined (DC offset and signal) components having a larger amplitude.This then causes the AGC loop to reduce the total gain such that thepower of the combined components is maintained at the AGC setpoint(e.g., I²+Q²=AGC setpoint). The reduced gain then causes compression ofthe desired signal components, with the amount of compression beingproportional to the magnitude of the DC offset. The smaller amplitudefor the desired signal components results in a degradedsignal-to-quantization-noise ratio (SNR_(Q)), which also degradesperformance. Moreover, if the DC loop is not able to completely removethe large DC offset before the it enters the tracking mode, then theresidual DC offset would be removed more slowly in the tracking mode.The AGC loop would then follow this slow transient response of the DCloop, which then results in a prolonged degradation period until the DCand AGC loops both achieve steady state.

Third, a large DC offset affects the ability to accurately detectjammers, which are interfering signals in the desired signal band. Ajammer may be generated by non-linearity in the circuits in the receivedsignal path. Since non-linearity in amplifier 114 and mixer 212 are morepronounced when these circuits are operated at high gains (i.e., morestages being turned ON), the receiver may detect for jammers right afterany of these circuits is switched to high gain. Jammer detection may beperformed by measuring the power of the signal components with RSSI 412right after the switch to a high gain, comparing the measured poweragainst a threshold after a particular measurement time period, anddeclaring the presence of a jammer in the signal components if themeasured power exceeds the threshold. If a jammer is detected, then thegain of one or more circuits may be reduced to either remove or mitigatethe jammer. However, in the presence of DC offset introduced by theswitch to the high gain, it may not be possible to discern whether theincrease in the measured power is due to jammer or to the total noise,which includes any unremoved DC offset and the increased DC loop noisegenerated by operating the DC loop in the acquisition mode to quicklyremove the DC offset. Thus, the presence of DC offset may impact theability to accurately detect for jammers, which may degrade performanceif the RF/analog circuits are operated at the wrong gains due toerroneous detection of jammers.

A large DC offset may cause long bursts of errors due to variousdeleterious effects described above. The degradation due to DC offset ismore problematic at higher data rates since the time needed to remove DCspikes may be fixed (e.g., by the specific design of the DC loop), whichthen results in more errors at higher data rates.

In accordance with another aspect of the invention, the duration of timethe DC loop is operated in the acquisition mode is inverselyproportional to the bandwidth of the DC loop in the acquisition mode.The DC loop bandwidth is designed to be wider in the acquisition mode toallow the DC loop to more quickly respond to and remove DC offset.Increasingly wider loop bandwidth corresponds to increasingly fasterloop response. As noted above, DC error in the desired signal componentsmanifests as noise after the despreading operation within digitaldemodulator 144. This noise should be removed as quickly as possible,which may be achieved by increasing the bandwidth of the DC loop for theacquisition mode. However, the wider DC loop bandwidth also results inincreased DC loop noise that may also degrade performance.

To maximize performance, the acquisition mode should trade off betweenthe (introduced) DC offset to be corrected and the (self-generated) DCloop noise. To limit the amount of DC loop noise and still allow the DCloop to operate at high bandwidth, the time duration in which the DCloop is operated in the acquisition mode may be set inverselyproportional to the loop bandwidth. A wider DC loop bandwidth generallycorresponds to a shorter DC offset acquisition time due to the widerloop's ability to more quickly respond. Thus, the shorter amount of timespent in the acquisition mode with the wider DC loop bandwidth takesadvantage of this fact, and the DC loop is not operated in theacquisition mode for longer than necessary, which may then improveperformance.

The specific time duration to operate the DC loop in the acquisitionmode may also be selected based on various other factors such as, forexample, the expected amplitude of the DC offset, the amplitude of theDC loop noise, the modulation schemes, the bandwidth of the receivedsignal, and so on. In general, the acquisition mode duration isinversely related to the DC loop bandwidth in the acquisition mode, withthe exact function being dependent on the factors noted above.

In accordance with yet another aspect of the invention, the operation ofthe AGC loop is made dependent on the DC loop's operating mode. As notedabove, any un-removed DC offset, which is typically larger when the DCloop changes into acquisition mode, affects the operation of the AGCloop. Thus, DC offset canceller 130 provides to AGC loop unit 142 theDC_loop_mode control signal, which indicates the DC loop's currentoperating mode. When the DC loop is switched to the acquisition mode toquickly remove a (potentially) large DC offset, the AGC loop maysimultaneously be switched to either the low gain mode or the freezemode so that the AGC loop responds slowly or not at all to the DC offsetwhile the DC loop is in the acquisition mode. The AGC loop may then beswitched back to the normal mode after the DC loop transitions to thetracking mode.

The small or zero AGC gain used while the DC loop is in the acquisitionmode ensures that the AGC loop preserves its control signals during theDC acquisition period. The AGC control signals will then be ready tooperate in the normal manner once the DC loop enters the tracking mode.The small or zero AGC gain also hinders or prevents the AGC loop frommoving the power of the desired signal components from the AGC setpoint,and further reduces the impact of the DC offset in the jammer detectionprocess, which would then reduce the likelihood of erroneous jammerdetection.

The specific normal and small AGC gains to be used may be determined bysimulation, empirical measurement, or some other means. These gains mayalso be programmable (e.g., by controller 160).

Serial Bus Interface (SBI)

In accordance with yet another aspect of the invention, the controls forsome or all of the RF/analog circuits are provided via serial bus 152.The use of a standard serial bus to control RF/analog functions providesmany advantages, as described below. Moreover, the serial bus may bedesigned with various features to more effectively provide the requiredcontrols, as also described below.

Conventionally, controls for RF/analog circuits (e.g., amplifier 114 andmixer 212) are provided using dedicated signals between the circuits tobe controlled and the controller providing the controls. One or morepins may be designated on the controller for each circuit to beindividually controlled. For example, three pins may be designated onthe controller and the RF/analog chip to control the five stages of theamplifier/mixer described above. The use of designated pins for specificfunctions increases pin count and complicates board layout, which maylead to increased cost for the receiver.

The use of a serial bus to provide controls for RF/analog circuits canameliorate many of the disadvantages encountered in the conventionaldesign and can further provide additional benefits. First, the serialbus can be implemented with few pins (e.g., two or three) and these samepins can be used to provide control for multiple circuits implemented inone or more integrated circuits (ICs). For example, a single serial busmay be used to control the gain of amplifier 114, the gain of mixer 212,the DC offset of mixer 212, the frequency of oscillator 218, and so on.By reducing the number of required pins to interconnect the RF/analog ICwith the controller, the costs of the RF/analog IC, the controller, andthe circuit board may all be reduced. Second, the use of a standardserial bus increases flexibility for future chip sets since itstandardizes the hardware interface between the RF/analog IC and thecontroller. This also allows a manufacturer to use the same board layoutwith different RF/analog ICs and/or controllers without altering orincreasing the number of required control lines.

In an embodiment, SBI unit 150 is designed to support a number ofhardware request (HW_REQ) channels, each of which may be used to supporta particular function. For example, one channel may be used for the VGAloop to set the step gains of amplifier 114 and mixer 212, and anotherchannel may be used for the DC loop to set the DC offset control value(DC1) for mixer 212. In general, the SBI unit may be designed to supportany number of hardware request channels.

Each circuit to be separately controlled may be associated with arespective address. Each message transmitted via the SBI unit includesthe address of the circuit for which the message is transmitted. Eachcircuit coupled to the serial bus would then examine the addressincluded in each transmitted message to determine whether or not themessage is intended for that circuit, and would only process the messageif it is addressed to that circuit.

In an embodiment, each hardware request channel may be designed with theability to support a number of data transfer modes, which may include afast transfer mode (FTM), an interrupt transfer mode (ITM), and a burstor bulk transfer mode (BTM). The fast transfer mode may be used totransmit multiple bytes to multiple circuits in accordance with thefollowing the pattern: ID, ADDR, DATA, ADDR, DATA, . . . where ID is thehardware request channel ID, ADDR is the address of the recipientcircuit, and DATA is the data for the recipient circuit. The interrupttransfer mode may be used to transmit a single byte for broadcasting toone or more circuits coupled to the serial bus. And the burst transfermode may be used to transmit multiple bytes to a specific circuit inaccordance with the following the pattern: ID, ADDR, DATA1, DATA2, . . .. Different and/or additional transfer modes may also be implemented,and this is within the scope of the invention.

In an embodiment, the hardware request channels may be assigned specificpriorities (e.g., by the controller). The priorities of the channels maybe programmed into a register within SBI unit 150. The channels'priorities would then determine the order in which messages are sent, ifmultiple messages need to be sent by the SBI unit over the serial bus. Ahigher priority may be assigned to a channel used for a control looprequiring fast response (e.g., the gain step for amplifier 114 and mixer212) and a lower priority may be assigned to a channel used for morestatic functions (e.g., the receive mode of direct downconverter 120,e.g., DFM and GPS).

Each hardware request channel may also be associated with a respectiveenable flag that indicates whether or not that channel is enabled foruse. The enable flags for all channels may be maintained by SBI unit150.

In an embodiment, the serial bus comprises three signals—a data signal,a clock signal, and a strobe signal. The data signal is used to send themessages. The clock signal is provided by the sender (e.g., thecontroller) and used by the receivers to latch the data provided on thedata signal. And the strobe signal is used to indicate the start/stop ofmessages. Different serial bus designs with different signals and/ordifferent number of signals may also be implemented, and this is withinthe scope of the invention.

The various aspects and embodiments of the direct downconversionreceiver described herein may be implemented in various wirelesscommunication systems, such as CDMA systems, GPS systems, digital FM(DFM) systems, and so on. The direct downconversion receiver may also beused for the forward link or the reverse link in these communicationsystems.

The various aspects and embodiments of the direct downconversionreceiver described herein may be implemented by various means. Forexample, all or some portions of the direct downconversion receiver maybe implemented in hardware, software, or a combination thereof. For ahardware implementation, the DVGA, DC offset correction, gain control,SBI, and so on may be implemented within one or more applicationspecific integrated circuits (ASICs), digital signal processors (DSPs),digital signal processing devices (DSPDs), programmable logic devices(PLDs), field programmable gate arrays (FPGAs), processors, controllers,micro-controllers, microprocessors, other electronic units designed toperform the functions described herein, or a combination thereof.

For a software implementation, the elements used for gain control and/orDC offset correction may be implemented with modules (e.g., procedures,functions, and so on) that perform the functions described herein. Thesoftware codes may be stored in a memory unit (e.g., memory 162 inFIG. 1) and executed by a processor (e.g., controller 160). The memoryunit may be implemented within the processor or external to theprocessor, in which case it can be communicatively coupled to theprocessor via various means as is known in the art.

Headings are included herein for reference and to aid in locatingcertain sections. These headings are not intended to limit the scope ofthe concepts described therein under, and these concepts may haveapplicability in other sections throughout the entire specification.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

The invention claimed is:
 1. An automatic gain control (AGC) apparatusoperable during direct conversion of RF signals comprising: an analogvariable gain amplifier for amplifying with a gain control value derivedat least in part from a determined value of a DC offset of the baseband;a digital variable gain amplifier coupled to an output of the analogvariable gain amplifier; a gain controller adapted to measure a signalstrength and to control the gains of the analog and digital variablegain amplifiers; and a DC offset canceller interposed between the outputof the analog variable gain amplifier and an input of the digitalvariable gain amplifier, the DC offset canceller operative in aplurality of possible DC operating modes including an acquisition modeand a tracking mode, wherein the acquisition mode has a wider loopbandwidth than that of the tracking mode and is used to more quicklyremove a large DC offset in the signal.
 2. The automatic gain control(AGC) apparatus operable during direct conversion of RF signalsaccording to claim 1, wherein an AGC loop gain is varied according to anoperating mode of the DC offset canceller.
 3. The automatic gain control(AGC) apparatus operable during direct conversion of RF signalsaccording to claim 2, comprising: a first integrated circuit comprising:said digital variable gain amplifier; said gain controller adapted tomeasure said signal strength and to generate a first gain control signalto control the gain of the digital variable gain amplifier and a secondgain control signal, the first and second gain control signalscontrolling components of an AGC loop gain; and a second integratedcircuit comprising: said analog variable gain amplifier, receiving thesecond gain control signal and varying gain in accordance therewith, theoutput of the analog variable gain amplifier coupled to the input of thedigital variable gain amplifier.
 4. The first integrated circuit,operable with a second integrated circuit to form the automatic gaincontrol (AGC) apparatus operable during direct conversion of RF signalsaccording to claim 2, the second integrated circuit comprising theanalog variable gain amplifier, receiving a first gain control signaland varying gain in accordance therewith, and a DC offset cancellerinterposed between the output of the analog variable gain amplifier andan input of the digital variable gain amplifier, the first integratedcircuit comprising: the gain controller adapted to measure a signalstrength and to generate a second gain control signal to control thegain of the digital variable gain amplifier and the first gain controlsignal, the first and second gain control signals controlling componentsof an AGC loop gain.
 5. The first integrated circuit, operable with asecond integrated circuit to form the automatic gain control (AGC)apparatus operable during direct conversion of RF signals according toclaim 2, the second integrated circuit comprising the digital variablegain amplifier, the gain controller adapted to measure a signal strengthand to generate a first gain control signal to control the gain of thedigital variable gain amplifier and a second gain control signal, thefirst and second gain control signals controlling components of an AGCloop gain, the first integrated circuit comprising: the analog variablegain amplifier, receiving the second gain control signal and varyinggain in accordance therewith, the output of the analog variable gainamplifier operable for coupling to the input of the digital variablegain amplifier.
 6. The automatic gain control (AGC) apparatus operableduring direct conversion of RF signals according to claim 1, wherein thegain between the output of the analog variable gain amplifier and theinput of the digital variable gain amplifier is essentially constant. 7.A method of operating an automatic gain control (AGC) loop incombination with a DC loop comprising: selecting a particular DCoperating mode for the DC loop from among a plurality of possible DCoperating modes that include an acquisition mode operable for aparticular time duration inversely proportional across a loop bandwidthfor the DC loop for the acquisition mode; operating the DC loop in theselected DC operating mode to correct for DC offset in a signal;selecting a particular AGC operating mode for the AGC loop from among aplurality of possible AGC operating modes based on the selected DCoperating mode; and operating the AGC loop in the selected AGC operatingmode to provide variable gain for the signal, the variable gainincluding control for RF circuitry derived at least in part from adetermined value of a DC offset in a baseband of the signal, wherein theplurality of possible DC operating modes include the acquisition modeand a tracking mode, and wherein the acquisition mode has a wider loopbandwidth than that of the tracking mode and is used to more quicklyremove a large DC offset in the signal.
 8. The method of claim 7,wherein each of the plurality of possible AGC operating modes isassociated with a respective AGC loop gain.
 9. The method of claim 7,wherein the plurality of possible AGC operating modes includes a normalmode and a low gain mode.
 10. The method of claim 9, wherein theplurality of possible AGC operating modes further include a freeze mode.11. The method of claim 7 wherein the selected AGC operating mode is alow gain mode when the selected DC operating mode is the acquisitionmode.
 12. The method of claim 7 wherein the selected AGC operating modeis a freeze mode when the selected DC operating mode is the acquisitionmode.
 13. A direct conversion receiver unit in a wireless communicationsystem, comprising: a DC loop configurable to operate in one of aplurality of possible DC operating modes that include an acquisitionmode operable for a particular time duration inversely proportionalacross a loop bandwidth for the DC loop for the acquisition mode tocorrect for DC offset in a signal; and an automatic gain control (AGC)loop configurable to operate in one of a plurality of possible AGCoperating modes to provide variable gain for the signal, wherein theparticular AGC operating mode to be used is determined based on theparticular DC operating mode selected for use for the DC loop, thevariable gain including control for RF circuitry derived at least inpart from a determined value of a DC offset in a baseband of the signal,wherein the plurality of possible DC operating modes include theacquisition mode and a tracking mode, and wherein the acquisition modehas a wider loop bandwidth than that of the tracking mode and is used tomore quickly remove a large DC offset in the signal.
 14. The directconversion receiver unit in a wireless communication system according toclaim 13, comprising: a control apparatus adapted for use in the directconversion receiver, comprising: means for selecting the particular DCoperating mode for the DC loop from among the plurality of possible DCoperating modes; means for operating the DC loop in the selected DCoperating mode to correct for DC offset in a signal; means for selectingthe particular AGC operating mode for the automatic gain control (AGC)loop from among the plurality of possible AGC operating modes based onthe selected DC operating mode; and means for operating the AGC loop inthe selected AGC operating mode to provide variable gain for the signal.15. The direct conversion receiver unit according to claim 14, whereinsaid means for selecting a particular DC operating mode is a DC offsetcanceller.
 16. An apparatus, comprising: means for selecting aparticular operating mode for a DC loop in a receiver unit from among aplurality of possible operating modes that include an acquisition mode;and means for, if the selected operating mode is the acquisition mode,operating the DC loop in the acquisition mode for a particular timeduration to correct for DC offset in a desired signal, wherein theparticular time duration is inversely proportional across a loopbandwidth for the DC loop for the acquisition mode, and transitioningout of the acquisition mode after the particular time duration into atracking mode, wherein the acquisition mode has a wider loop bandwidththan that of the tracking mode and is used to more quickly remove alarge DC offset in the signal.
 17. The apparatus according to claim 16,further comprising: means for selecting the acquisition mode in responseto an event expected to result in a large DC offset in the desiredsignal, where the event corresponds to application of a new DC offsetvalue to correct for static DC offset in the desired signal.
 18. Theapparatus according to claim 16, wherein the transition being made fromthe acquisition mode to the tracking mode after the particular timeduration.
 19. The apparatus according to claim 16, further comprisingmeans for selecting the acquisition mode in response to an eventexpected to result in a large DC offset in the desired signal, where theevent corresponds to a switch to new analog circuit stages to processthe desired signal.
 20. A method, operable with a DC loop in a receiverunit, the DC loop comprising a first integrated circuit, the firstintegrated circuit comprising an analog variable gain amplifier and a DCoffset canceller, and a second integrated circuit, the second integratedcircuit comprising a digital variable gain amplifier, and a gaincontroller adapted to measure a signal strength and to control the gainsof the analog and variable digital variable gain amplifiers, the methodcomprising: selecting a particular operating mode for the firstintegrated circuit from among a plurality of possible operating modesthat include an acquisition mode; and if the selected operating mode isthe acquisition mode, operating the first integrated circuit in theacquisition mode for a particular time duration to correct for DC offsetin a desired signal, wherein the particular time duration is inverselyproportional across a loop bandwidth for the DC loop for the acquisitionmode, and transitioning out of the acquisition mode after the particulartime duration into a tracking mode, wherein the acquisition mode has awider loop bandwidth than that of the tracking mode and is used to morequickly remove a large DC offset in the signal.
 21. The method accordingto claim 20, the acquisition mode being selected in response to an eventexpected to result in a large DC offset in the desired signal, where theevent corresponds to application of a new DC offset value to correct forstatic DC offset in the desired signal.
 22. The method according toclaim 20, wherein the transition being made from the acquisition mode tothe tracking mode after the particular time duration.
 23. The methodaccording to claim 20, the acquisition mode being selected in responseto an event expected to result in a large DC offset in the desiredsignal, where the event corresponds to a switch to new analog circuitstages to process the desired signal.